1. Field of the Invention
This invention relates to a zero voltage switching solid state relay. The term "solid state relay" as herein used is intended to refer to a solid state switching circuit which ignites a semiconductor controlled rectifier element, such as a photothyristor, thyristor or light-AC controlled element, by means of an external control signal, thereby to feed a load with an alternating current.
An example of such a solid state relay is a solid state switching circuit using photon coupling. In such a solid state switching circuit using photon coupling, a light emitting element, such as a light emitting diode, is caused to emit light in response to an external control signal, and the light is received by a light receptor, such as photothyristor, thereby to ignite the photothyristor and feed a load with an alternating current.
2. Description of the Prior Art
The zero voltage switching solid state relay of the type to which this invention is directed will be explained by taking as an example a solid state switching circuit of the photon coupling type which has been employed in the prior art, such as described in U.S. Pat. No. 3,816,763 to Korn et al. When a predetermined input electric signal is applied across the input terminals of a light emitting diode, the diode emits light in a quantity corresponding to the magnitude of the input electric signal. A photothyristor disposed opposite to the light emitting diode receives the light emitted thereby and is biased into the conductive state. When the photothyristor has become conductive, a load, an AC power source, the photothyristor and a diode bridge which are connected to output terminals of the solid state switching circuit constitute a closed circuit, so that a current flows through the load.
The solid state switching circuit is provided with a zero voltage switching circuit which consists of a transistor and a resistor connected to the gate of the photothyristor. The photothyristor is ignited only when the voltage of the AC power source is near zero, and it does not operate when the voltage is great. Accordingly, in the case where the light entering from the light emitting diode is of sufficient magnitude, and when the voltage of the AC power source is near zero, the photothyristor is ignited to permit the load current to flow through the load, so that the zero voltage function is performed.
The zero voltage switching circuit effectively operates also in point of the withstand quantity of voltage build-up rate (dV/dt). More spectifically, when an electrical noise is externally superposed on the AC power source, the photothyristor is ignited by the electrical noise. However, when the transistor in the zero voltage switching circuit is rendered conductive by the electrical noise, the photothyristor becomes difficult to ignite. Therefore, the withstand dV/dt of the circuit is sharply enhanced.
Accordingly, the zero voltage switching solid state relay of the prior art is capable of providing the zero voltage switching function and the high withstand dV/dt. Disadvantageously, however, it has no hysteresis characteristic in the switching operation.
More specifically, the zero voltage switching solid state relay of the prior art has its conductive and nonconductive states inverted by a certain point of value of the input control signal. The conductive and nonconductive states are therefore inverted by a small change of the input signal or a change of the ambient temperature, so that the stability is unsatisfactory.